Uber 2019 revenue $14.15 billion, divided by your estimate of 10 million cars is $1415 per car per year. I would suggest you're estimating at least an order of magnitude too high for the car count, since they can more or less operate 24x7.

That would bring the capex down below $100 billion - less than Google's cash on hand. And you'd be starting with the most lucrative cities while staggering it out, and have a very good idea of your payback by the time you've spent serious cash.

Technology and regulatory risks are high for Waymo, but capex is very manageable.